Electrochemical evaluation of resistance to localised corrosion of vitreous coatings containing particles applied on metallic substrates for biomedical applications

CrCoMo alloys are generally known to be highly resistant to corrosion-induced failures and to spontaneously form passive oxide films when immersed in physiological environments. The stresses resulting from the use of the prosthesis can cause fracture and abrasion of the oxide film that covers the surface. This can result in crevice corrosion, with the corresponding decrease in pH leading to severe attack. Protective coatings deposited onto the metals would minimise the release of metallic ions from the substrate and the incorporation of bioactive particles would enable the adhesion to the bone tissue. This work describes the electrochemical behavior of CrCoMo alloys (F75) as cast, covered by a hybrid silica coating obtained by sol-gel. The silica sol-gel coating contains bioactive glass ceramic and vitreous particles of the system CaO-SiO2-P2O5 previously obtained by fusion, crystallization and milling. The performance of the alloy with and without coating is evaluated in simulated body fluid (SBF) pH 7.31 at 37 degreesC. As cobalt base alloys are susceptible to crevice attack, coated pieces are also tested in SBF acid solution (pH 0.7) in order to simulate the acidic media and the high chloride concentrations that develop when a crevice is formed. The electrochemical behavior has been evaluated by potenciodynamic polarization and electrochemical impedance spectroscopy. It was found that for both pH conditions the samples coated with double layer of glass ceramic particles showed lower passive current densities than those with monolayers. The coating improves the protection potential of the base material in the neutral solution. Besides, in the acidic solution, all the samples remain passive but the protection potential shifts to potentials more active than those measured in the neutral SBF. (C) 2002 Elsevier Science B.V. All rights reserved.